Leaking Trans Fluid

Hello folks,I have a 1994 Chevy Z-71 4x4 auto with a 350 throttle body,The truck has 54,000 original miles. recently on a three hour trip it leaked a quart of trans fluid which covered the underside of the truck,it does not leak when sitting only when driving.

Check the cooler lines running up to the radiator, the overflow vent, then the dipstick, then the gasket around the pan.
I'd buy some cleaner and hose everything down first to get it as clean as you can, after hosing it down put the rear wheels in the air on jackstands and block the front wheels, then fire it up and put it in gear let is run and see if you can find the leak.

My first guess would be a leaking cooler line, if it's coming out the dipstick or vent then you've got a heat or pressure problem, could come out the dipstick if the vent is clogged, and the last item would be a bad pan gasket which given the age of the truck (even though it is low miles) wouldnt be out of order. Sitting around and not driving is bad on the seals and gaskets.

Were you towing or hauling a heavy load when it happened? Sometimes these transmissions leak fluid out the breather on the top of the transmission case when towing and the fluid gets hot. Also, how hard were you running the truck? (lots of hard acceleration and such)

Crawdaddy, was pulling an empty trailer and the cruise control set on 70 mph, no excess acceleration.Thanks for your reply. Gonna try to get my old self underneath in the morning and clean things up and see what I can find.

I spent most of the day trying to find a leak from the trans. I cleaned off everything underneath and drove around some and crawled back under and could not find any leak.The cooler lines are good,nothing coming from fill tube,couldn't see anything from the vent tube. I am going to a mechanic friend tomorrow to put on a lift and let him look.Maybe he can find the problem.

I spent most of the day trying to find a leak from the trans. I cleaned off everything underneath and drove around some and crawled back under and could not find any leak.The cooler lines are good,nothing coming from fill tube,couldn't see anything from the vent tube. I am going to a mechanic friend tomorrow to put on a lift and let him look.Maybe he can find the problem.

Thanks for all the advice.

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Maybe from the overflow tube at the top of the tranny. Mine's blowing fluid from the overflow tube which is why I mention it. I've yet to address it because other than being messy it's benign (so far). It started doing this one random day; there was no obvious cause.

Thanks brother SurrrealOne, I ran my hands all over the top of the trans and transfer case and found no fluid.I am flustered.

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Mechanic checked it for me today and found a slight seeping from the cooler lines and stated the vent tube was wet and that's where the fluid came from,but couldn't tell me why . What causes the pressure to blow the fluid from the vent tube????